In August 1980, with no hope left of winning the nomination, Sen. Edward M. Kennedy conceded defeat to incumbent Jimmy Carter in the Democratic presidential race.

"For me, a few hours ago, this campaign came to an end," Kennedy said at the Democratic National Convention in New York. "For all those whose cares have been our concern, the work goes on, the cause endures, the hope still lives, and the dream shall never die."

And with that, at age 48, Kennedy returned to the Senate, where he committed himself to a career as a legislator, crafting landmark bills on health care, education and immigration. Many Democrats are now pointing to the Kennedy model as a path for Sen. Hillary Rodham Clinton to reshape her own political career, assuming she is unable to wrest the nomination from Sen. Barack Obama.

"I loved the Senate before I ran for the president," Kennedy explained in an interview before his recent cancer diagnosis. Losing to Carter, he said, made him appreciate the opportunities in Congress all the more. "I think I became a better senator, with greater focus and attention," Kennedy said. But he added: "It all depends on the attitude, what's in the mind of the person."

Clinton, Kennedy continued, must decide where her heart lies. "She's got great capacity -- she was a good senator before, and she can be a great senator in the future," he said. The question, he said, is "what she does with this experience."

When Kennedy returned to Capitol Hill before the 1980 election, the Massachusetts Democrat was in a similar fix. Like Clinton, he was the heir to a powerful political legacy. But the climate was volatile, and voters were in the mood for change. Kennedy was rejected by many of his Senate colleagues, despite Carter's sagging popularity, and he won just 10 primary states. But like Clinton, he hung on until the bitter end.

Yet Kennedy was an 18-year Senate veteran who had already risen to chairman of the Judiciary Committee and a health subcommittee. Clinton faces few options for quick advancement should she give up her presidential bid, prompting some to speculate that she may look elsewhere for a prominent political post, possibly the governorship of New York.

The climate on Capitol Hill has changed considerably in the 18 months since Clinton began her presidential campaign. The Senate leadership path that she had once viewed as a viable alternative is now all but blocked. Senate Majority Leader Harry M. Reid (Nev.) has gained clout in his role, and he will grow even more powerful if Democrats succeed in expanding their narrow majority in November by up to half a dozen seats.

"Within the caucus, there's strong support for Senator Reid, and those who speculate otherwise don't understand the Senate," said Durbin, who was the first senator to endorse Obama. When Clinton returns to her old job, assuming she does not win the nomination, Durbin added, "she will be an important part of the future. But I can't tell you that anyone has approached me, or anyone in the caucus, with any specific suggestions about what she would do."

When Clinton announced her bid in January 2007, she was the prohibitive favorite, and most of her Senate colleagues appeared ready to rally to her side. But as her primary battle with Obama draws to an end, with the senator from Illinois almost certain to emerge the victor, Clinton has discovered that the reservoir of Senate goodwill was not so deep after all.

Clinton collected 13 endorsements from her Senate colleagues, compared with 15 for Obama, and she has not added a name to her list since early February, even though she has won significant contests since then.

"I'm sure she'll remember, for the rest of her life, who was with her and who wasn't," said Sen. Christopher J. Dodd (D-Conn.), who ran unsuccessfully this year and then endorsed Obama.

Speaking on the condition of anonymity, many Democratic senators said they expect Clinton to work doggedly for Obama this summer and fall, and they agreed that if she does, whatever hard feelings that linger from the primary race will vanish.

But a bigger question is whether, like Kennedy, she will shelve her presidential ambitions, especially if Sen. John McCain (R-Ariz.) wins in November. The 2012 election would coincide with the end of Clinton's second Senate term, effectively turning her into a lame duck. A run for New York governor would hasten Clinton's departure by two years.

But if Obama wins in November, her next likely opportunity for the presidency would be in 2016, when she would be 69. If Clinton makes it clear her future is in the Senate, she could find several paths open to her, aides and colleagues said.

One would be to champion a major piece of legislation, such as the health-care bill Obama has promised early in his first term.

A member of three prominent committees, Clinton remains a junior member on all three panels and does not stand to become a committee chairman for at least another decade.

But another option would be to assume the chairmanship of the Democratic Senatorial Campaign Committee, a demanding but high-profile post that is an appointment by Reid. Sen. Robert Menendez (N.J.) is a potential successor to Schumer, who has led the committee for four years, but Democratic sources said Clinton could get the job if she wanted it.

Sen. Patrick J. Leahy (Vt.) pointed to the late Hubert H. Humphrey (D-Minn.) as one example of life in the Senate after a losing White House bid. A senator in the 1950s and '60s, Humphrey became vice president in 1965 and then narrowly lost to Richard M. Nixon in the 1968 presidential election. He won another Senate term in 1970 and returned as the most junior member. "He realized he could command an audience anywhere in the world. He threw himself into the issues. He had the time of his life," Leahy said.

On the other hand, Sen. Joseph R. Biden Jr. (Del.) returned to the Senate after his failed 1988 presidential bid and became a formidable voice on both the Judiciary and Foreign Relations committees.

With or without a prominent post, Clinton will possess unrivaled clout, her colleagues said. "She is the single most powerful woman in America, and that will be solidified by this race, not diminished by it," said Biden, who has not endorsed a candidate after dropping his own bid earlier this year.

As the former first lady, Clinton arrived in the Senate in January 2001 already a political celebrity, and her status was acknowledged with an appointed leadership position as head of the Steering Committee, with the task of interacting with outside liberal groups.

But colleagues said Clinton showed no interest in using her perch to work toward more powerful posts inside the Senate. Rather, she spent much of her time traveling the country to help Democrats in presidential battleground states, and raising money through her leadership political action committee, HillPAC. She also committed herself to advancing New York state interests, numerous colleagues and senior aides said.

Regardless of which route she now chooses, colleagues who have run failed campaigns said she must first readjust to life in the Senate.

"When you're out on the campaign, you've got to make decisions every hour, every minute," said Sen. Tom Harkin (D-Iowa). "Then you come back to the Senate and it's like a cocoon."